This invention relates to a system and a method for increasing the efficiency and for improving the control of gas turbines and associated gas booster systems.
Gas booster systems for gas turbine (e.g. microturbine) systems are well known. A conventional gas turbine system typically includes a gas booster system to supply pressurized gaseous fuel to a combustion chamber that generates high temperature exhaust for the turbine. Various parameters have been used to control the flow rate and pressure of the fuel being supplied to the gas turbine engine. An example of a conventional system is disclosed in U.S. Pat. No. 4,178,754. This system includes a positive displacement gear type fuel pump. The fuel pressure is controlled by bypassing flow using a pressure regulator. The fuel flow rate is controlled using a metering valve. The regulator and the metering valve operate in response to turbine operating conditions as represented by the following parameters: turbine inlet temperature, compressor inlet temperature, and compressor exit pressure. Similarly, in U.S. Pat. No. 5,850,733, which is incorporated herein by reference, the fuel flow to a gas turbine is controlled, in part, based on the gas turbine exhaust temperature.
In some conventional gas turbine systems, a reciprocating type compressor is used to compress the natural gas being used as fuel in the gas turbine combustion engine. These reciprocating compressors normally operate at a fixed rate of displacement. Accordingly, to regulate the fuel flow rate into the compressor, a system of valves is opened or closed in response to various system parameters. These systems are generally disfavored because the number of valves required in such a system require many pipe connections, resulting in an increased risk of leaks due to the intense vibrations generated by reciprocating compressors. Moreover, such systems include a significant lag-time between the time when the parameter is detected and the time when the system adequately responds by opening or closing one of the valves in the system.
U.S. Pat. No. 5,606,853 describes a system in which the rate of compression in a screw type compressor can be changed using a variable speed motor. By changing the compression rate, the fuel flow rate into the combustion chamber can be adjusted thereby eliminating the need for a system of valves which are opened and closed in response to a measured system parameter. The speed of the motor driving the compressor is dependant on a computer signal generated in response to a measurement of the power output of the turbine. However, measurements of turbine power output do not provide as accurate a measure of system conditions as do measurements of the turbine temperature. Specifically, fuel demand is more accurately determined by measuring turbine inlet and exhaust temperatures. Thus, there remains a need to provide a gas booster system that automatically supplies the appropriate amount of fuel at the appropriate pressure to the combustion chamber based on either the turbine inlet or exhaust temperature.
The present invention addresses this need and provides other benefits as described further below.
The system provides a gas turbine system comprising: a pump for providing a gaseous fuel to a combustion chamber which produces an exhaust; an electric motor which drives the pump; a turbine which receives the exhaust from the combustion chamber; a sensor which measures a parameter of the temperature of the turbine; and a controller which adjusts the speed of said electric motor in response to the turbine temperature.
Preferably, the pump is a positive displacement pump, and among positive displace pumps it is ideally a rotary screw pump. Additionally, the sensor is preferably a thermocouple. The sensor may be positioned to measure the turbine temperature at an inlet or at an outlet of the turbine. The sensor may measure the turbine temperature by measuring the temperature of the combustion chamber exhaust before it enters the turbine inlet or by measuring the temperature of turbine exhaust which exits through the turbine outlet. Alternatively, the sensor may measure the turbine temperature by measuring the temperature of a metal part of the turbine inlet or outlet.
The controller controls the speed of the variable speed motor by sending a control signal, which is responsive to the temperature measured by the sensor, to a variable frequency drive. The variable frequency drive, in turn, adjusts the speed of the variable speed motor in response to the control signal sent by the controller. The control signal from the controller may be generated in numerous ways. In a first embodiment, a difference between the turbine temperature and a predetermined second temperature may be determined by the controller. A control signal corresponding to the difference between the temperatures may then be sent to the variable frequency drive which, in turn, adjusts the speed of the variable speed motor.
In this first embodiment, if the sensor is positioned at the turbine inlet, the predetermined second temperature may be between 1400xc2x0 and 1700xc2x0 F., and is preferably approximately 1600xc2x0 F. If, on the other hand, the sensor is positioned at the turbine outlet, the predetermined second temperature may be between 1175xc2x0 and 1550xc2x0 F.
In a second embodiment, the controller may send a control signal which is responsive to the measured turbine temperature (rather than to a difference between the turbine temperature and a predetermined second temperature) to the variable frequency drive. The variable frequency drive may then adjust the speed of the variable speed motor in response to the control signal.
Finally, the system may include a hermetically sealed passage connecting the pump to the combustion chamber. In this case, the hermetic seal ideally comprises a magnetic coupling.
A method for controlling a gas turbine system is also contemplated. The method includes: compressing a gaseous fuel with a pump controlled by an electric motor; forcing the compressed gaseous fuel into a combustion chamber; igniting the compressed gaseous fuel in the combustion chamber and thereby producing an exhaust which enters a turbine; measuring a temperature of the turbine; and controlling the speed of the electric motor in response to the turbine temperature.
In following this method, the step of compressing of the gaseous fuel is preferably completed by a positive displacement pump, and among positive displacement pumps it is ideally a rotary screw pump. The step of measuring the temperature of the turbine includes positioning a sensor, which is preferably a thermocouple, either at an inlet or at an out of the turbine. Moreover, the step of measuring the turbine temperature, contemplates either measuring the temperature of the combustion chamber exhaust before the exhaust enters the turbine through the turbine inlet or measuring the temperature of an exhaust which exits the turbine through the turbine outlet. In the alternative, the step of measuring the turbine temperature can be completed by measuring the temperature of a metal part of either the turbine inlet or the turbine outlet.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only, and are not restrictive of the invention as claimed.